/Engine/Source/Runtime/Engine/Classes/AI/Navigation/NavigationTypes.h
New non Legacy NavLinkIds have the most significant bit set to 1. Only Id's formed fromGenerateUniqueId()should have the most significant bit set to 1. We should only see extremely rare clashes here anyway with 63 bits for the city hash in the sort of number of CustomLinks we are likely to have and there is a system in place to handle this for dynamic meshes at run time. For static meshes the AuxiliaryId / ActorInstanceGuid will need to be generated again in the editor (probably easiest by deleting and readding the actor / component in the editor at the moment).
bool IsLegacyId() const